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External Form.—It would be quite impossible for any one to
confuse any other quadrupedal animal with a mammal. The
body of a reptile is, as it were, slung between its limbs, like the
body of an eighteenth century chariot between its four wheels;
in the mammal the body is raised entirely above, and is
supported by, the four limbs. The axes of these limbs too, as a
general rule, are parallel with the vertical axis of the body of
their possessor. There is thus a greater perfection of the
relations of the limbs to the trunk from the point of view of a
terrestrial creature, which has to use those limbs for rapid move-
ment. The same perfection in these relations is to be seen, it
should be observed, in such running forms among the lower
Vertebrata as the Birds and the Dinosaurs, where the actual
angulation of the limbs is as in the purely running Mam-
malia. These relations are of course absolutely lost in the
aquatic Cetaeea, and not marked in various burrowing creatures.
The way in which the fore- and hind-limbs are angulated is
considerably different in the two cases. In the latter, which
are most used and, as it were, push on the anterior part of
the body, the femur lias its lower end directed forwards, the
tibia and the fibula project backwards at the lower end, while
the ankle and foot are again inclined in the same direction as
the femur. With the fore - limbs there is not this regular
alternation. The humerus is directed backwards, the fore-arm
forwards, and the hand still more forwards. This angulation
seems to facilitate movement, inasmuch as it is seen in even the
Amphibia and the lower Eeptilen, in which, however, the differ-
ences between the fore- and hind-limbs are less marked, indicat-